Not Applicable
1. Field of the Invention
The present invention relates to a simplified method for measuring the flow rate of a fluid in which the propagation times of ultrasonic signals transmitted through the fluid can be detected to determine fluid flow rate. The invention further relates to improvement of a probe flow sensor configuration and to its installation, and similar sensing devices.
2. Background Information
Transit-time ultrasonic flow sensors, also known as xe2x80x9ctime-of-flightxe2x80x9d ultrasonic flow sensors, detect the acoustic propagation time difference between the upstream and downstream ultrasonic transmissions in a moving fluid and process this information to derive a fluid flow rate. Several different sensor configurations have been used including: 1) direct measurement of the propagation time of a pulse emitted by a first transducer and received by a second transducer where the change in time is a function of fluid flow rate; 2) dual xe2x80x98sing-aroundxe2x80x99 sound velocimeters, where the difference in xe2x80x9csing-aroundxe2x80x9d frequency between the velocimeters is a function of the fluid flow rate; 3) sensors producing continuous waves using two widely different high frequency carriers but commonly modulated with another much lower frequency signal where the phase difference of the modulated signal on the received carriers is a function of the fluid flow rate; and 4) sensors producing bursts of continuous waves using a single frequency on a pair of transducers, the burst duration being less than the acoustic propagation time between the transducers, where the time difference between the received transmissions is a function of flow rate.
The transducers of transit-time ultrasonic flow sensors are most often field mounted, and are commonly individually attached to the outside of a pipe, thereby offering the advantage of not having to break into the pipe in order to make the flow measurement. However, the uncertainty of the pipe wall integrity and the effects of its surface condition, and the uncertainties of locating, attaching and acoustically coupling the transducers to the pipe, as well as uncertainties of the reflection from the interior of the pipe when it is used to complete the acoustic path between the transducers, can often lead to substantial measurement error. Even when the transducers are in contact with the fluid being measured (i.e., wetted), their mechanical location may result in misalignment, being spaced at the wrong distance or set at the wrong angle, all of which can result in measurement error. As a result, these sensors are usually equipped with supporting electronics containing sophisticated diagnostic means for confirming proper installation and operation. Overall, these sensors are relatively expensive and have a reputation for sometimes producing erroneous measurements.
A notable example of prior art in this area is U.S. Pat. No. 4,221,128 to Lawson, who teaches an acoustic flow meter in which bursts of acoustic energy are periodically and simultaneously emitted by each of two transducers. The bursts are shorter than a transit time between the two transducers and a portion of the acoustic energy from each transducer is received by the other of the two transducers. Because of the fluid flow, there is a relative phase shift between the two received signals. This phase shift is measured by beating each of the received signals against a common reference signal to produce signals at a lower frequency at which the phase difference is more easily measured by means of electronic circuitry that is less expensive, consumes less power, and is more stable than older prior art equipment. The prior art methods cited by Lawson directly measured the phase difference at the higher frequency selected for acoustic transmission.
Further teaching in this area is provided by the inventor in U.S. patent application Ser. No. 09/592,313, filed on Jun. 13, 2000. In this application the inventor teaches a transit time ultrasonic flow sensor having two simultaneously transmitting transducers. This sensor is configured to compensate for circuit-related drifts in the flow rate output signal. During acoustic transmission some of the transmitting signal is also routed through the receiving circuits to generate a reference signal that is used to compensate the received signals. The disclosure of U.S. patent application Ser. No. 09/592,313 is incorporated herein by reference.
It is an object of the present invention to provide means for reducing the complexity and cost of transit-time ultrasonic flow sensors, to improve their measurement reliability, and to make their installation easier.
It is a further object of the present invention to provide a cost effective means for enabling transit-time flow sensors to measure a fluid flow rate along two axes.
The above and other objects may be satisfied with a transit-time flow sensor configured as a single modular unit as exemplified in accordance with some preferred embodiments of the present invention. One of the preferred embodiments includes an insertion probe with two permanently mounted transducers which enters a relatively small opening in a pipe carrying a flowing fluid. A probe-mounted acoustically reflective surface is also provided to enable the acoustic path to be completed within the entity of the probe so that it does not depend upon any other reflective surface for its operation. This sensor is thus more simple in construction, easier to install correctly, and provides more reliable operation. The probe may of course be supplied factory mounted and calibrated in a short section of pipe, and thereby be considered a xe2x80x9cfull borexe2x80x9d sensor for installation between two similar pipe sections.
In an embodiment of the invention configured as a probe, the transducers are spaced out along the flow direction so that one is upstream relative to the other. These transducers are in line with and at an angle to the fluid flow, and are directly wetted by the fluid. In this configuration the sensor is isolated from the attenuation and multipath problems which occur when the transducers are pipe mounted. The supporting electronics may be simplified in concept and incorporate cost effective components while still offering good measurement precision.
One method of flow rate sensing used with the present invention differs from the four methods listed in the foregoing xe2x80x98Discussion of Prior Artxe2x80x9d, in that it uses a variable frequency acoustic signal which is continuously transmitted by either one or the other transducers as they alternate between transmitting and receiving states. In this arrangement, a relatively low alternation frequency is the exclusive modulation source and the primary detection of time difference occurs at the transmitted acoustic frequency without using an intermediate frequency. This method, in its basic form, is unstable because of both acoustic path and electronic related drifts and frequency-related uncertainties. Special frequency control provisions have been employed to correct these deficiencies so that high flow sensing sensitivity, good zero stability and low noise level are obtained.
In a second method of flow rate sensing used with the of the present invention, the variable frequency acoustic signal is simultaneously transmitted by both transducers in a burst. After an interval corresponding to the expected transit time between transducers, both transducers are switched to their respective receiving states and a phase difference between their received signals is used as a measure of the fluid flow rate.
The invention provides transmission modes, both continuous or in bursts, and means of providing operating stability by controlling the acoustic frequency, that are also applicable to xe2x80x9cfull borexe2x80x9d sensors where the transducers are individually mounted rather than being configured as a probe. Furthermore, when using the burst mode flow sensing may be performed with the acoustic energy being conveyed through the pipe wall on which the transducers are mounted.
In yet other preferred embodiments, two pairs of transducers are used within a single sensor housing and are located to determine fluid flow rate components along two orthogonal axes,
In further embodiments, an electric current is passed between surfaces of the sensor in the proximity of the acoustic path so that, by the process of electrolysis, those surfaces will be maintained clean.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and who wishes to learn how to practice the invention, it will be recognized that the foregoing recital is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages may be provided by some embodiments.